Natural and Dammed Rivers: Which is Better for Salmon’s Summer?

Freshwater habitats are experiencing a consistent increase in water temperature in recent years, challenging particularly cold-water species. For this reason, juveniles of Atlantic salmon, Salmo salar, can experience temperature outside their thermal optimum (i.e., > 20°C) or even temperature causing them thermal stress (i.e., >23°C). It is demonstrated that particular river patches, the so called thermal refugia (e.g., tree shaded areas, morphological features), can represent an important buffer against these thermal peaks. It is also well known that dams alter river water temperature; specifically, the release of thermally stratified water from deep-release dams makes dammed rivers cooler in summer and warmer in winter when compared to natural rivers. However, due to their other ecological harms, dammed rivers are usually not considered for their potential benefits as thermal refugia. We studied a confluence site between a natural and a dammed river in Scotland (situated in the upper part of the Tay River catchment) throughout 2021. We collected water temperature (kinematic temperature-T_k) values using loggers (temporal data) and an Unmanned Aerial Vehicle (UAV)-mounted thermal camera to produce high resolution T_k maps of the site (with average Mean Absolute Error =-0.28°C ± 0.5 °C). The first aim of our work is to evaluate water T_k spatial distributions and temporal trends during summer to verify whether the dammed river can consistently offer a cooler thermal habitat for juvenile salmon. We found that, in summer, the natural river T_k reached daily T_k>20°C and can frequently pass the 23°C threshold, while the dammed river rarely passed 20°C; accordingly, T_k map for summer 2021 confirms that the dammed river offers a spatially-consistent cooler thermal habitat (T_k < 20°C at a T_k daily maximum time). Following these results, we investigated for the same study area T_k spatial distribution during the spawning season (late autumn 2021). It is known that the timing for adult salmon females’ eggs deposition depend from water temperature in nests site area, happening earlier in cooler water (i.e. upper part of rivers catchment) and later in warmer water (i.e. lower catchment area). We found that in early winter, the time frame for egg laying, the dammed river is consistently warmer than the natural river of 2°C in average. Considering these findings along with the well-known territorial behavior of juveniles Atlantic Salmon, we discuss if the potential summer thermal suitability of dammed river is canceled out by the winter T_k alteration, making them unsuitable river patches for eggs deposition.